[sdiy] voltage regulator insulation

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Hello Guys,

I'm currently building a +/-15V, 1A PSU for my  dual Mega Percussive 
Synth and some other drum voices.
I use an LM317 and LM337. I want to use the metal case as heatsink.
Now I read somewhere that you need to put an insulation plate between 
the metal case and the LM's.
Is this really needed? What about the bolt that I'll use to hold the LM's?
And where do I get this insulation plate? What kind of material is it?

And a related question, which ground do I connect the case with? Do I 
use the DC ground or the mains ground.

Thanks a bunch for your help!

Cheers,
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